In 3G or beyond 3G mobile system infrastructure, a typical network topology of base stations includes a baseband unit (BBU) and a plurality of remote radio units (RRUs). The RRUs are placed near the antenna as the final end close to the air interface, and coupled to the BBU to convert a baseband signal to an RF signal, or vice versa. Typically, the RRUs have some analog circuitries for radio power transmission and some digital circuitries for evaluating performance of the RRUs and supporting the analog circuitries.
Digital circuitries of an RRU may perform transceiver interfacing, pulse shaping, up/down frequency converting, peaks clipping, and most importantly, digital pre-distortion (DPD) to compensate power amplifier (PA)'s non-linearity and memory effects, etc. The performance evaluation includes measurements of transmit power, transmit power efficiency, Extended Master Session Key (EMSK), adjacent channel leakage ratio (ACLR), error vector magnitude (EVM), etc.
The adjacent channel leakage is always a concern for the RRU, which is caused by non-linearity of the PA where odd-order intermodulation distortion (IMD) components are introduced into the desired signal band. To compensate the non-linearity of the PA, a DPD module is operable to pre-distort an input signal of the PA in accordance with the measured ACLR, so that the odd-order of IMD components are at least partially canceled. Thus, the ACLR is an important transmit spectrum shape measurement figure.
Before delivery of the RRU, it is subjected to a test so as to meet radio requirements. The test includes measurement of the ACLR for pre-verifying the DPD module. However, the bothering issue is that the ACLR would deteriorate due to many reasons and potential risks in an application environment.
One approach widely known in the art to implement monitoring performance in the application environment is to use internal coefficients updating status of the DPD module to indicate the RRU performance. The approach is based on two assumptions: one is that DPD output validity is the only factor that influences the DPD performance; the other is that the DPD output validity and the DPD performance are determined by the internal coefficients updating status. In other words, an asserted internal coefficients updating status ensures a valid DPD output and a good DPD performance. However, the fact is that the two assumptions are not always tenable.
It is still desirable to implement monitoring performance of the RRU in the application environment in cost-efficient and reliable manner, and report to a user or take recovering actions when the above problem occurs.